This invention relates to a jet mill in which a high-speed air flow is supplied into a grinding chamber of a hollow part in the interior of the mill body through air nozzles inclined on the outer wall so that the coarser powder (the material to be ground) is continuously ground to fine particles (fine powder) of micron-order size in the grinding chamber while at the same time classification is effected by the swirling air flow.
In jet mills, a high-speed air flow is supplied into the grinding chamber through the air nozzles inclined on the outer wall so that the coarser material to be ground is reduced in size as it swirls and classification is also effected by the swirling air flow. Jet mills are known as grinding devices suitable for yielding superfine products (particles). They are characterized in that the interior of the grinding chamber is simple in configuration, the top and bottom surfaces of the grinding chamber are easy to separate and reassemble, and cleaning can easily be done both before and after operation.
On the other hand, the jet mill relies solely upon the air flow to grind the material in the grinding chamber, so it is difficult to grind the material to a specified particle size or control the particle size distribution to a narrow enough range. Various improvements have been attempted with a view to grinding the material to a specified particle size or narrowing the particle size distribution; three known approaches are air nozzles that permit adjustment of the angle at which the air flow is injected into the grinding chamber to enable control of the ground particle size distribution over a wide range (see, for example, JP 52-44450 A, in particular, pages 3-4 and FIGS. 2-3), a special classification mechanism such as a classification rotor provided around an exit pipe to improve classification precision (see, for example, JP 63-319067 A, in particular, pages 2-3 and FIGS. 1-3), and impact members in, for example, spherical, cylindrical or hemispherical form provided inside the grinding chamber against which the material to be ground is caused to collide with the air flow to achieve the higher grinding efficiency in the grinding chamber (see, for example, JP 57-84756 A, in particular, page 2 and FIGS. 2-3; JP 4-210252 A, in particular, pages 2-5 and FIGS. 1-2; and JP 6-254427 A, in particular, pages 3-6 and FIGS. 1-2).
However, although enabling control of the ground particle size distribution over a wide range, the swirling fluid-energy mill disclosed in JP 52-44450 A suffers from a problem of a poor classification precision, since the mill injects compressed air to grind a material and at the same time forms swirling flow to perform classification, thereby also ejecting yet large particles.
And, the horizontal swirling flow jet mill disclosed in JP 63-319067 A, while resolving the problem of a poor classification precision of the mill of JP 52-44450 A, has problems such as that a turbulence is generated in swirl and that fine particles adhere to the rotor wall, due to a difference in speed between the swirling flow formed by the compressed air and that formed by the classification rotor.
Furthermore, the jet mills disclosed in JP 57-84756 A, JP 4-210252 A and JP 6-254427 A, while improving the grinding efficiency, have problems such as that the impact member obstructs the air flow, generating a significant turbulence in the swirling flow to thereby lower the classification precision or to allow the ground material to heavily adhere to the impact member, resulting in difficulty with a stable (continuous) operation.
Moreover, these prior arts all require that complexly shaped mechanical parts be additionally provided inside the grinding chamber, as exemplified by the special mechanism for adjusting the angle at which the air flow is injected, the special mechanism for classification, and the special impact members provided inside the grinding chamber. These compromise the three advantageous features of the jet mill, that is, the interior of the grinding chamber is simple in configuration, the top and bottom surfaces of the grinding chamber are easy to separate and reassemble, and cleaning can easily be done both before and after operation. Therefore, the aforementioned prior arts have not been completely satisfactory.